Abstract/Summary

A geochemical and geophysical reconnaissance
programme was commenced in 1978 to investigate
the favourability of the large Carboniferous depositional
trough underlying Northumberland for the
discovery of new metalliferous mineral deposits
and/or new styles of mineralisation. The long
historical association of the southern part of
the area with successful mining ventures
indicated that a more comprehensive evaluation of
potential was justified. The importance of the
Whin Sill as a host lithology had long been
recognised and the disposition of the Sill played a
significant part in determining the direction of
effort in the area.
The association of mineral deposits with faulting
in the Whin Sill was considered to justify an
airborne magnetic survey for mapping such
structures in that part of the basin underlain by the
sill at shallow depth. The main geochemical effort
involved a drainage reconnaissance of the entire
Lower Carboniferous trough area.
The geochemical data obtained from stream
sediments and panned concentrates were processed
bY computer, employing simple statistical
techniques from which a number of anomalous
areas were defined which are not attributable
to either the known ore bodies or artificial
contamination. The data derived from the regional
geochemical survey identified not only the known
(and now largely worked out) mining areas but also
a number of other areas with anomalously high
metal values. Barium, in stream sediments and
panned concentrates, was a reliable indicator of
mineralisation and identified the Settlingstones-
Whinnetley-Fallowfield area. High barium values
in concentrates were also obtained from an area
to the south of Rothbury (Ewesley Farm) which
is geographically remote from any known mineralisation.
Soil samples subsequently collected in this
arca also contained elevated barium values, and it is
considered that unexposed barium mineralisation
exists in the area, probably associated with a
fracture cutting the Whin Sill.
From the airborne geophysical data a number
of linear magnetic anomalies were identified,
several of which can be equated with known fault
structures or their probable extensions. Of the
faults thus indicated, some have carried significant
mineralisation, and apparently related magnetic
anomalies in their vicinity were thus identified as
of possible mineral exploration significance.
1
Several of these linear magnetic anomalies were
further examined by geochemical (soil sampling)
and ground-geophysical techniques. The geochemical
data obtained from the soil traverses in
the areas examined did not provide unequivocal
information, values for the ore elements being
generally low.
One magnetic anomaly indicating an eastward
extension of the Sun Vein near to
Newbrough was identified as a drilling target,
and four inclined boreholes were drilled from two
sites to test the fault structure affecting the Whin
Sill as interpreted from the magnetic data. Three
of the holes were continued to sufficient depth to
pass through the Whin Sill and into the sediments
beneath. Sufficient information was obtained from
the holes to permit stratigraphic correlation
between them and also to establish structural
relationships, while considerable variation in the
texture and degree of alteration of the quartzdolerite
was apparent in the cores. Base metal
mineralisation associated with the alteration of the
Sill, and also in some of the carbonate sediments,
was identified.
Chemical analyses of samples from the Whin
Sill quantify the changes in composition effected
by the process of hydrothermal alteration.
Magnetic susceptibility values determined on
the Whin Sill core show great variability, consistent
with the variation in the degree of alteration to
White Whin.